1. Field of the Invention
The invention relates to a transmission system with a superconductive cable having three phase conductors and a cryostat surrounding the phase conductors and encasing a hollow space for conducting a cooling agent, wherein a neutral conductor is present common to the three phase conductors, and wherein the cryostat is made of a circumferentially enclosed, thermally insulated sheath.
Such a transmission system is disclosed in WO 2004/013868 A2.
2. Description of the Prior Art
Superconductive cables have in today's technology electrical conductors of a composite material which contains ceramic material, which at sufficiently low temperatures changes over into the superconducting state. The electrical direct current resistance of an appropriately constructed conductor is zero with sufficient cooling as long as a certain current is not exceeded. Suitable ceramic materials are, for example, BSCCO (Bismuth-strontium-calcium-copper-oxide) as material of the first generation, or ReBCO (Rare-earth-barium-copper-oxide), particularly YBCO (Yttrium-barium-copper-oxide), as materials of the second generation. Sufficiently low temperatures for placing such materials into the superconducting state are, for example, between 67 K and 90 K. Suitable cooling agents are, for example, nitrogen, helium, neon and hydrogen or mixtures of these materials.
EP 2 017 856 A1 describes a superconductive electrical cable which has a conductor constructed as conductor rope of superconductive wires. The conductor is surrounded by a dielectric which is enclosed by a screen, or a return conductor, or a neutral conductor. The vein of the cable formed in this manner is arranged in a cryostat which consists of two pipes of metal arranged concentrically relative to each other and including a vacuum insulation. It is also possible to arrange three such veins in a cryostat for forming a three-phase system. The conductor can also be arranged directly in a cryostat which is surrounded by a dielectric and a screen.
In the known transmission system according to the above-mentioned WO 2004/013868 A2, the three-phase conductors are insulated relative to each other and arranged concentrically relative to each other. The neutral conductor is arranged with the inclusion of another insulating layer as fourth conductor concentrically around the three-phase conductor. The compact arrangement with four conductors of superconducting material in a cryostat is placed in a cryostat and is cooled during operation of the transmission system until the superconductivity is reached. The cryostat has dimensions whose sizes are adapted to this compact arrangement, including a sufficiently large hollow space for conducting the cooling agent therethrough.